1. Field of the Invention
The present invention relates to an oscillator circuit, a PLL circuit, a semiconductor chip, and a test apparatus. Particularly, the present invention relates to an oscillator circuit that generates an oscillation signal with low phase noise and a PLL circuit a semiconductor chip and a test apparatus that employ the oscillator circuit.
2. Related Art
A voltage controlled oscillator (VCO) has been known as a circuit that generates an oscillation signal. The voltage controlled oscillator is an oscillator in which the frequency of an oscillation signal is controlled corresponding to an input control voltage. For example, the voltage controlled oscillator generates an oscillation signal with a frequency corresponding to an input voltage by circularly connecting a plurality of inverters in which the delay time is changed according to the input control voltage.
However, since the voltage controlled oscillator has the loop described above, a phase noise component generated in a previous cycle is accumulated in a subsequent cycle. Therefore, it has had a problem that a voltage controlled oscillator generates an oscillation signal with a large phase noise component.
Additionally, it is difficult for a semiconductor chip and a test apparatus employing the voltage controlled oscillator to accurately operate because the phase noise component of the oscillation signal generated by the voltage controlled oscillator is large.
Generally, it has been considered that the phase noise generated in the voltage controlled oscillator is reduced by employing the voltage controlled oscillator in a PLL circuit. However, an oscillation signal is synchronized with a reference signal corresponding to an edge of the provided reference signal in the PLL circuit. Therefore, the accumulated phase noise component is cut every time the edge of the reference signal is provided. However, the phase noise component can not be removed while the PLL circuit self-oscillates between the edges of the reference signal. Moreover, the jitter component of the reference signal for the PLL circuit causes the oscillation signal to generate jitter.
Thus, an advantage of the present invention is to provide an oscillator circuit, a PLL circuit, a semiconductor chip, and a test apparatus which are capable of solving the problem accompanying the conventional art. The above and other advantages can be achieved by combining the features recited in independent claims. Then, dependent claims define further effective specific example of the present invention.